SCS is a type of neurostimulation primarily intended to manage chronic pain, particularly within the back, neck, arms or legs. Benefits of SCS or other forms of neurostimulation may include: a reduction in pain; a reduction or elimination of the use of pain medications; and increased activity levels and an improved overall quality of life. Neurostimulation has been used to manage pain from failed back surgery syndrome or post-laminectomy syndrome and other neuropathies. To this end, an SCS system may be implanted within the body to deliver electrical pulses to nerves along the spinal cord. Some patients describe the resulting sensation as a gentle massaging sensation or, in some cases, simply the absence of pain. The SCS system typically includes a small generator device similar to a pacemaker but equipped to send electrical pulses to leads mounted along the nerves near the spinal cord. The generator is usually implanted in the abdomen or buttock area. The stimulation leads may include, e.g., thin wires or paddles for delivering electrical pulses to the nerves along the spinal cord. Thin wire leads, also referred to as percutaneous leads, may be implanted within the epidural space. Paddle leads are instead typically implanted during a surgical procedure where a small amount of bone is removed from one of the vertebra. An external controller, similar to a remote control device, is provided to allow the patient to control or adjust the neurostimulation.
SCS devices and other neurostimulators may be programmed or controlled using one or more stimulation sets or “Stim Sets.” The stimulation sets specify the particular electrodes to be used as cathodes and anodes, as well as the pulse amplitude, pulse width and pulse frequency, and may further specify the duration and timing of the stimulation (i.e., the “dosage” specified, e.g., as “continuously running vs. a specified on/off cycle” or specified as a duration of a bolus followed by a minimum refractory period before the next bolus). SCS devices typically use a single stimulation set to capture neural structures at one anatomic location, or multiple stimulation sets that run simultaneously (interleaved) to achieve stimulation at multiple neural structures/anatomic locations. For example, programming an SCS system to cover a complex pain syndrome may require two stimulation sets to adequately treat knee and thigh pain, a third stimulation set to treat hip pain, and fourth and fifth stimulation sets to treat bilateral low back pain. Further, multiple stimulation sets may be used to adequately cover a broader region of pain without causing undesirable collateral stimulation of non-painful neighboring regions.
FIG. 1 illustrates the effect of a pair of stimulation sets (Stim Set 1 and Stim Set 2) configured to deliver interleaved sequences of biphasic pulses (sequences 2 and 4) using multiple electrodes of a percutaneous lead 5 to achieve stimulation at both a middle location 6 and a distal location 8 along the lead in accordance with at least some prior art techniques. In the figure, pulse sequence 2 generated by the first stimulation set is shown in sold lines along with the vectors used for that set. Pulse sequence 4 generated by the second stimulation set is shown in phantom lines along with the vectors used for that set. For Stim Set 1, pulses are either delivered between the two most distal of the electrodes of the lead or between a distal electrode and a middle electrode. In contrast, for Stim Set 2, pulses are either delivered between two of the middle electrodes or between one middle electrode and a distal electrode. In this manner, cathodic stimulation is achieved both at the distal end 8 of the lead and at the middle location 6, albeit with the complexity of requiring two stimulation sets. Note that, alternatively, the SCS device might try to exploit both anodal and cathodal stimulation (with cathodal stimulation at a distal electrode and anodal stimulation at a middle electrode) but anodal stimulation thresholds are two to three times higher than cathodal thresholds for neural stimulation. As such, to achieve anodal stimulation to deliver dual-site capture, much higher pulse amplitudes would be required, draining power and potentially causing unwanted accessory stimulation.
To provide cathodic stimulation to additional locations along the lead, more stimulation sets can be employed to specify additional and more densely interleaved pulse sequences. In addition to requiring more complex programming by the user (i.e. the clinician), multiple stimulation sets will tend to draw increased current from the implantable pulse generator (IPG) battery. In this regard, as the number of interleaved pulses increases so does the effective frequency from the IPG's single current source. As frequency increases, the device switches from passive discharge to active discharge, further draining energy while not contributing to additional neurophysiologic effects. These issues may become even more problematic within neurostimulation systems used to treat cardiac disorders such as heart failure or arrhythmia (such as SCS to reduce heart rate or blood pressure, protect against ischemic signaling transduction outside of ischemic areas, modulate coronary circulation, modulate local and regional refractoriness, etc.) To achieve the needed stimulation at various locations along the cervical and thoracic spine to achieve these cardioprotective effects, still more complex SCS stimulation sets may be required, resulting in even greater current drain and programming complexity.
Accordingly, it would be desirable to provide techniques for achieving efficient multi-site neurostimulation at reduced current drain. It is to these ends that aspects of the invention are generally directed.